its a calculated risk because no one knows the exact specie of fungus that is growing on a particular cake.. it can be benign or bad..
i personally will not touch any "moldy" tea.. and have largely decreased my intake of poorly stored/shu pu-erh as well.
if the mold on the tea's from the famed eurotium species, it might be perfectly safe with the tea flavour enhanced by fungal metabolites. but if its from the other family of "common yellow molds" i.e. aspergillus, you run into the risk of the presence of certain toxic aspergilli species that secrete aflatoxin. (incidentally the chinese name for aspergilli type of molds 黄曲霉 literally translates as "yellow mold")
citing an article from last year : http://news.cntv.cn/china/20120325/117033.shtml
of which the title translates as : can pu-er cause cancer?
in July, 2011, there was a study carried out in collaboration between Guangzhou City Cancer Therapeutic centre and Zhong Shan University/Southern Medical University on pu-er teas. 70 pu-er tea samples were obtained and analysed, with 11% of them exceeding the national "limit" for aflatoxin content.
the reporter called the principal investigator, professor Zhu Wei (Guangzhou Cancer therapeutic centre, toxicology dept)
reporter : how much was the aflatoxin limit exceeded by?
zhuwei : not very much i suppose, because there is actually no national limit/restriction that was placed on pu-er tea. so we just used the limit/standard that is currently enforced on candy/sugar products which is 5 milligrams per kilogram. in our 70 samples, 8 of them had a level that exceeded this limit.
no one seem to test pu-er tea that stringently for pesticides, toxins and all that so far..
the only way around this is to have the mold tested, either by PCR or some toxin detection assay, or to brew the tea in a "tibetan" like manner, cooking the tea (sometimes boiling the h3ll out of the leaves) in which aflatoxin can potentially be inactivated.
unless this tea is so extremely tasty that one has to take a risk/chance to drink it (just like fugu fish), i personally wouldnt recommend the consumption.